Organic EL devices are known to be highly efficient and are capable of producing a wide range of colors. Useful applications such as flat-panel displays have been contemplated. Representative of earlier organic EL devices are Gurnee et al U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; Dresner, "Double Injection Electroluminescence in Anthracene," RCA Review, Vol. 30, pp. 322-334, 1969; and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. Typical organic emitting materials were formed of a conjugated organic host material and a conjugated organic activating agent having condensed benzene rings. The organic emitting material was present as a single layer medium having a thickness much above 1 micrometer. Thus, this organic EL medium was highly resistive and the EL device required an extremely high voltage (&gt;100 volts) to operate.
The most recent discoveries in the art of organic EL device construction have resulted in devices having the organic EL medium consisting of extremely thin layers (&lt;1.0 micrometer in combined thickness) separating the anode and cathode. The thin organic EL medium offers reduced resistance, permitting higher current densities for a given level of electrical bias voltage. In a basic two-layer EL device structure, one organic layer is specifically chosen to inject and transport holes and the other organic layer is specifically chosen to inject and transport electrons. The interface between the two layers provides an efficient site for the recombination of the injected hole-electron pair and resultant electroluminescence. Examples are provided by U.S. Pat. Nos. 4,356,429; 4,539,507; 4,720,432; 4,885,211; 4,950,950; 5,047,687; 5,059,861; 5,061,569; 5,073,446; 5,141,671; 5,150,006 and 5,151,629.
The simple structure can be modified to a three-layer structure, in which an additional luminescent layer is introduced between the hole and electron transporting layers to function primarily as the site for hole-electron recombination and thus electroluminescence. In this respect, the functions of the individual organic layers are distinct and can therefore be optimized independently. Thus, the luminescent or recombination layer can be chosen to have a desirable EL color as well as a high luminance efficiency. Likewise, the electron and hole transport layers can be optimized primarily for the carrier transport property.
One inherent drawback of the organic EL devices is that the mobility of electrons in organic materials is extremely low, so that a high voltage is required to produce a strong electric field. For instance, the mobility of electrons in Alq is in the range of 10.sup.-6 -10.sup.-7 cm.sup.2 /V.multidot.S, and thus a field of 1.times.10.sup.6 V/cm is necessary for efficient electron transport. The thickness of the organic medium can be reduced to lower the voltage level required for device operation, however, the reduction results in low quantum efficiency due to the effect of radiative quenching by a conducting surface, high leakage current, or device shorting.